Customize: Consider using more-specific "locked" messages when branching is enabled

[dlh]

When changeset branching is enabled, users can be locked out of editing individual changesets instead of the whole Customizer.

However, the "locked" message when branching is enabled still refers to another user "customizing this site," potentially implying that the Customizer itself is inaccessible.

The attached patch would attempt to provide different "locked" messages when branching is enabled that limit the scope of what they tell the user.

[westonruter]

Good point.

[westonruter]

When in branching mode, would it be useful to have a button that opens the Customzier with a new changeset? That may be premature. Eventually there should maybe be a way to navigate to a list of all changesets to pick from.

[dlh]

My initial reaction is also that a "Create" button might be premature. Plus, if I'm reading correctly, _admin_notice_post_locked() doesn't offer a button to go to the "Add New" screen, in which case adding one in the Customizer would create an inconsistency.

42655.2.diff​ reduces the variables into an array of strings so more can be added easily later.

[dd32]

Bumping to 4.9.3 due to 4.9.2s release

[westonruter]

In 42538:

Customize: Use changeset-specific "locked" messages when branching is enabled.

Props dlh.
See #39896.
Fixes #42655.

[westonruter]

In 42539:

Customize: Use changeset-specific "locked" messages when branching is enabled.

Props dlh.
See #39896.
Fixes #42655 for 4.9 branch.